Solid Oxide Fuel Cells (SOFCs) have been researched for operation on anaerobic digester (AD)-derived biogas at wastewater treatment plants (WWTPs). SOFCs can perform well on light hydrocarbon fuels and the use of AD-derived biogas provides an opportunity for biogas to be used as a renewable fuel.
Tests were conducted at three levels of H2 dilution (using N2, Ar and CO2 as diluent gases, plus H2O) to examine the performance of tubular SOFCs. When gases that are inert in SOFC reactions are used there is a decrease in cell performance. When CO2 was used the decrease in cell performance was higher due to the reverse water-gas shift (WGS) reaction reducing the partial pressure of H2. A computer simulation was developed to predict SOFC system efficiency and GHG emissions. The simulation indicated that the system electrical efficiency is higher for a S/C ratio of 2 then a S/C ratio of 1 due to the increased partial pressure of H2 in the reformate. The reduction in GHG emissions is estimated to be approximately 2,400 tonnes CO2, 60 kg CH4 and 18 kg N2O, annually.
Testing was conducted using a simulated biogas reformate mixture that was developed through a research initiative in which urban areas with populations over 150,000 in the United States and 50,000 in Canada were solicited to participate in a survey of biogas composition. The biogas reformate composition was determined to be 66.7% H2, 16.1% CO, 16.5% CO2 and 0.7% N2, which was then humidified to 2.3 and 20 mol% H2O. The reformate tests conducted at the higher humidification level yielded a better performance than those at the lower humidification because the WGS reaction produced more H2 when additional H2O was provided.
It was concluded that WWTP AD-derived biogas, when cleaned effectively to remove H2S, Si compounds, halides and other contaminants, could be reformed to provide a clean, renewable fuel for SOFCs. Biogas/SOFC systems could provide electricity and heat to WWTPs, as well as reduce GHG emissions, thus reducing their energy costs and environmental impact.